This is the best shot I have to illustrate the internal ridges on the Bilstein coilover sleeves. These sleeves fit just about perfectly on a Bilstein shock, but on the Monroe shock bodies I used for this conversion I had to file the two ridges on each sleeve down .015 inch. That step made for a very snug fit, to the point that I don't know if I could separate the sleeve from the shock body.
The work is all done at this point, but you get the idea: the end of the sleeve rests where the top of the original spring perch used to be. The 7-inch length of the sleeve is perfect if you don't cut away too much of the shock body you use in the Koni conversion.
The D&D 2.5-inch spring perches fit just about perfectly inside the stock MN12 upper perches. As Dave said, premature wear could be an issue because much of the load is carried by this rubber perch, not the steel shock tower. Today I started making a template for an aluminum upper perch to replace the factory one, though a solid mount might have some drawbacks regarding the change of the angle of the shock shaft as the suspension compresses.
And here's how it looks installed in the car. The 10 inch springs are definitely the right length. By setting the springs up so that there's just enough tension to keep them in place at full suspension droop and virtually no preload, ride height was within 1/8 inch of where it was before this conversion.